Not Applicable
Not Applicable
The present invention is related to the field of optical communications networks employing optical amplifiers, and more particularly to methods of controlling the optical signal-to-noise (OSNR) ratios of optical channels in optical communications networks.
The use of wavelength division multiplexing (WDM) technologies and optical amplifiers has enabled the traffic-carrying capacity of optical communications networks to be increased without requiring the installation of new fibers. However, certain undesirable characteristics of optical amplifiers, for example erbium-doped fiber amplifiers (EDFAs), require special consideration to achieve desired network performance. One such characteristic is amplified spontaneous emission (ASE) noise generated by the amplifier during operation. The ASE noise mixes with the amplified optical signal, and reduces the ability of downstream circuitry to faithfully recover data from the signal. Additionally, the gain of EDFAs and other optical amplifiers is not constant across the band of wavelengths that constitute a typical WDM signal. Generally, some form of equalization of EDFA gain is required in order to provide adequate amplification of all wavelengths along the paths from the transmitters to the respective receivers.
Another factor complicating the design of optical communications networks is the need for very general network topologies, to provide maximum flexibility in meeting the communications needs of users. Many optical links today are point-to-point, xe2x80x9cbackbonexe2x80x9d segments of wide-area networks, such as the long-distance telephone system. In such applications, upgrading or re-configuring a link can be performed relatively easily, because all of the equipment that interfaces to a given end of the link is generally co-located. However, the increasing use of the Internet and other data services is creating a demand for more arbitrary optical network configurations. For example, organizations having a number of geographically separated sites may require a general mesh connection of optical links among the various sites. Performing an upgrade in networks with such arbitrary connections is more complicated than in point-to-point networks, because the various pieces of equipment that provide signals to a given link may not be co-located.
One known technique for equalizing transmission characteristics in optical networks is shown in U.S. Pat. No. 5,225,922 to Chraplyvy et al., entitled xe2x80x9cOptical Transmission System Equalizerxe2x80x9d. The output powers and signal-to-noise ratios of different optical channels are selectively equalized by measuring the signal-to-noise ratios of all signals received at one end of a multi-link transmission path, and iteratively adjusting the output powers of all transmitters that provide input signals to the path until the signal-to-noise ratio in each channel is within acceptable limits. The iterative nature of the equalization algorithm arises from the fact that a significant change in the signal power of a channel (such as when a new channel is added as part of an upgrade) affects the ASE in other channels. The channels must be adjusted together, and generally multiple times, to achieve the desired equalization across all channels.
While the technique of Chraplyvy et al. provides good equalization of a point-to-point WDM transmission path, its use in more arbitrary network configurations is generally infeasible. A network user adding or changing a given channel may not have access to the transmitters that provide the other optical signals existing on the path, and therefore cannot carry out the necessary re-adjustment of these transmitters to achieve acceptable signal-to-noise ratios in all channels. Additionally, the technique is relatively complex due to its reliance on iterated measurement and adjustment. It would be desirable to enable the upgrading or re-configuring of an optical communications network to obtain desired signal-to-noise ratios in both new and existing channels without requiring the readjustment of the transmitters of existing optical signals.
In accordance with the present invention, an optical communications network is disclosed in which the transmitter output power for a given channel can be adjusted to achieve a desired channel OSNR independent of the power levels of other optical signals carried on the same path. Channels can be added, dropped, or changed without the need for a complex equalization process and without requiring access to the transmitters for other signals existing on the same path.
In the disclosed network, the optical amplifier in each optical link extending between an optical transmitter and an optical receiver is configured to operate with constant gain over a specified range of input optical signal power, and is also configured such that the power level of the optical signal provided to each optical amplifier is within the specified range of input optical signal power. When a channel is being added or adjusted, the optical signal-to-noise ratio (OSNR) of the optical signal received at the receiver is measured, and the power of the signal transmitted by the transmitter is adjusted to attain a desired OSNR at the receiver. The OSNRs of other signals carried on the path are not affected, due to the constant-gain operation of the optical amplifiers, and therefore no adjustments of other transmitters are required. Upgrading or re-configuring paths in the network is generally much simpler than in prior optical networks. As a result, barriers to the use of arbitrary optical network configurations such as add/drop, ring, etc., are reduced, and the benefits of optical communications can be more widely enjoyed.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.